Crabs do not need to be isolated to molt, as long as they have enough space. When a tank is crowded, or if there isn't enough substrate, they can come across each other when digging. Three crabs in a ten gallon is okay as long as they are small, but they will quickly outgrow the space and become too crowded.
Here is the HCA guide to how many crabs and tank sizes depending on the crab sizes: